The specified invention represents an improvement of an existing process for the thermal separation of solvents from thermoplastic plastic materials, particularly elastomers. U.S. Pat. No. 3, 726,843 discloses a process of this kind for the separation of an alcane, particularly hexane, from ethylene propylene diene monomer (EPDM) rubber. U.S. Pat. No. 6,881,800 discloses a similar process with the difference that U.S. Pat. No. 3,726,843 demonstrates the thermodynamic states in clearer detail. These processes are based on similar separation techniques of polyethylene or polypropylene from hexane, which have been in use on an industrial scale for quite a long period of time.
The fundamental advantage of the mentioned processes lies in the fact that, during the process steps, the polymer is always present in dissolved form, as a melt or as a melt-type paste. This is achieved by adding an alcane to the monomer mixture during the polymerization step. Under a certain pressure, the monomers are polymerized in hexane at high degrees of conversion The solution is then, in addition, possibly decompressed and heated indirectly to a certain temperature using a single or a plurality of heat exchangers, wherein the pressure must be selected such that the polymer always remains nicely soluble inside the solvent to avoid the formation of deposits in the heat exchanger. In one example on an industrial scale with hexane as a solvent, the necessary pressure is approximately 50 to 80 bar (gauge), the necessary temperature 220 to 240° C. The solvent now undergoes flashing inside a separator to achieve 20 to 30 bar, wherein in the range slightly above critical a polymer-containing phase and a lighter, low-polymer phase form. These phases can then be separated by way of the difference in densities thereof. The temperature decrease in this pressure jump is minimal, because there is no enthalpy of vaporization in the above-critical range. The heat of the separated, low-polymer phase is therefore usable for heating the educt, which represents an essential advantage in terms of process technique. In a static flash vessel, the resulting polymer-rich phase undergoes further flashing to pressures between 1 bar (gauge) and 10 bar (gauge), wherein the pressure is selected such that the flash is as complete as possible, while the polymer-containing bottom, however, remains in the form of a melt. The bottom can be supplied to a degassing extruder or kneader by means of a polymer pump or a valve, inside which any remaining solvent and monomer residues are removed in an absolute or partial vacuum.
For the above process to work, a sufficient enthalpy gradient must be present such that, after flashing, the polymer remains in the form of a melt inside the two separators or the drawing-in of the degasser. This aspect limits said process, as many polymers have a maximum temperature before thermal degradation sets in that is below a range of 220 to 240° C. The use of a low-molecular solvent or of the monomer as a solvent is conceivable (in the presence of restricted solubility of the polymer in the monomer); however, in this case. the flash stage then results in temperatures that are far below the melting point. This problem, among others, is an issue with polystyrols or polybutadienes. While polybutadiene is not a thermoplastic material, it behaves, however, like a melt over a very restricted temperature range. In addition, with increasing molecular weight and copolymers, the melting point increases. This results in high torques, the formation of fine particles and reduced performance for higher-molecular EPDMs inside the degasser.
A further disadvantage of the existing process is the fact that the polymer-rich bottom is drawn from the flash container gravimetrically. If the viscosity of the bottom is too high, the pressure loss of the flow results in pressures that lead to strong cavitation of the bottom. This causes the transporting power of the supplied pump to be effectively limited. In high-molecular products. it was observed that the degasser downstream of the flash vessel suffers from operation-related problems, because the temperature of the bottom from the flash vessel drops due to the pressure jump, and the product tends to solidify. The product is then pulverized into particles by the shaft, which can plug up the exhaust vapor lines. Due to the fact that the heat transfer of the heating walls of the degasser is especially poor with high-molecular products, a considerable part of the volume is needed to heat the polymer particles above the plastification range at which point the shaft generates sufficient torque for heating the product by means of mechanical energy. This way, the degassing capacity of the degasser is substantially reduced, particularly in large-scale facilities, because the ratio of surface areas to volume becomes increasingly less favorable the larger the equipment size. Moreover, although the particles have a large surface, they do not dynamically change their surface areas, as is the case inside a melt. This further restricts the degasification capacity.